1. Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof, and particularly, to a MISFET having a lowered threshold voltage.
2. Description of the Related Art
To reduce power consumption of a MOSFET-LSI, conventionally, the power source voltage Vdd has been reduced more and more. The threshold voltage Vth of the MOSFET, however, has not been reduced much, in order to prevent increase of an OFF current. The drive performance Id of the transistor consequently tends to be lowered. A dynamic threshold voltage MOSFET (DTMOSFET) has been proposed as a device which overcomes the tendency (ref.: Fariborz Assaderaghi, et al, “Dynamic threshold-voltage MOSFET (DTMOS) for Ultra-Low voltage VLSI”, IEEE Trans. Electron Devices, vol. 44, pp.414–421, 1997).
The DTMOSFET is a MOSFET which electrically connects a gate with a well (which is an Si-body in an SOI substrate). This device has merits of a large driving performance and a small OFF current although the power source voltage Vdd is small. The ground that results these merits will be explained by an operational principle as follows, which is specific to the DTMOSFET. The gate voltage is transmitted to the substrate, generating a substrate bias effect. Accordingly, the threshold voltage Vth is low when the transistor is ON. When the transistor is off, the threshold voltage Vth is high.
This device has further merits as follows (ref.: A. Yagishita, et al., “Dynamic Threshold Voltage Damascene Metal Gate MOSFET (DT-DMG-MOS) with low threshold voltage, high drive current, and uniform electrical characteristics,” IEDM Tech. Dig., pp.663–666, 2000).
(1) The DTMOSFET has a small vertical electric field (in the direction vertical to channel surfaces) and large mobility. This is the reason why high driving performance is realized.
(2) The DTMOSFET fulfils an equation of dVg/dVch=1. Therefore, the S-factor is constantly about 60 mV/decade in areas where no short channel effect is generated. This is an ideal value (which is the best value at room temperature).
(3) Variants of the threshold voltage can be reduced (variants of the threshold voltage: ΔVth1 (DTMOSFET)<ΔVth2 (normal MOSFET)).
There is however a problem that the substrate bias coefficient γ (=dVth/dVbs) is difficult to increase.
As described above, the DTMOSFET has a problem that the substrate bias coefficient γ cannot be increased and the threshold voltage cannot be further reduced.